The assembly of the head of coliphage T4 is being studied as a model system for discovering mechanisms by which multimolecular structures are synthesized in cells. Since it is known that the T4 capsid is first assembled from precursor proteins to give a procapsid in which some proteins are then cleaved to give a final, stable phage head, our studies fall into two, related projects. The first is to purify a number of the major capsid precursor proteins, to determine their physical and chemical properties by analytical ultracentrifugation, circular dichroism, and amino acid analysis. These proteins, of which we have already purified P22, P23, P24, and the internal proteins, are to be used, together with phage DNA and bacterial membrane fractions in various combinations to examine the nature of the interactions between them using the techniques mentioned above plus electron microscopy to look for the formation of regular head-related structures in vitro. The second part of these studies is to characterize the conversion of the unstable procapsid to the phage head in molecular terms. This includes purifying P21 and characterizing the cleavage enzyme by analyzing the products of the cleavage reactions in vitro using amino acid analysis and end-group determination. We will also determine the effects of tertiary and quaternary structure on the cleavage reactions in vitro by examining the effects on cleavage of the state of organization of the precursor proteins and the presence of various additions to cleavage reaction mixtures. These additions will include combinations of purified head precursor proteins, bacterial membranes, and solvents already known to inhibit cleavage whose effects on tertiary structure can be studied.